High-purity silicon is used in semiconductor substrates and solar cells. As a method for producing such high-purity silicon, silica is melted in an arc furnace to obtain impurity-rich coarse silicon, after which the coarse silicon is hydrogenated into a silane (SiHCl3) gas, which is reduced with hydrogen to obtain high-purity silicon. A problem with this conventional method is that the process is complicated, failing to allow for quick production of high-purity silicon. In order to melt silica by an arc furnace, it is necessary to heat the material to 3000° C. by arcing, thereby requiring a high input energy for heating, resulting in a high production cost.
On the other hand, another method has been proposed in the art, in which a carbon source is used to reduce silicon oxide which has been purified through acid precipitation from an aqueous solution of silicon oxide dissolved in aqueous phase with an acidifying agent, thereby obtaining solar grade silicon, as described in Patent Document No. 1 (claim 1 of Patent Document No. 1). Patent Document No. 1 discloses that this reduction of purified silicon oxide into silicon is done in an arc furnace, a thermal reactor, an induction furnace, a rotary kiln and/or a microwave heating furnace (claim 7 of Patent Document No. 1).
Patent Document No. 2 discloses a method for producing silicon from silicon oxide through thermal reduction in a microwave oven. This conventional technique is a method in which silicon oxide is used as a starting material, and the obtained silicon is additionally subjected to a zone melting method in a microwave oven.    Patent Document No. 1: Japanese National Phase PCT Laid-Open Publication No. 2012-504100    Patent Document No. 2: Japanese National Phase PCT Laid-Open Publication No. 2011-500495